Department of Chemistry and Molecular Design Institute, New York University, 100 Washington Square East, Silver Center, Room 1001, New York, New York 10003, United States.
J Am Chem Soc. 2015 Apr 22;137(15):5177-83. doi: 10.1021/jacs.5b01763. Epub 2015 Apr 10.
Optical rotations and rotatory strengths are calculated for achiral, conjugated hydrocarbons with the aim of determining to what extent the sum-over-π → π* rotatory strengths are sufficient to account for nonresonant optical activity. The separability of σ and π electrons might provide a short cut to the interpretation of chiroptical structure-property relations in some cases. It is shown that by restricting the analyses to planar, C(2v)-symmetric π-systems and their one electron HOMO-LUMO excitations, an intuitive understanding of the vexing property of optical activity is forthcoming for the following reasons: Hückel wave functions are simply calculated, and in some cases, they can even be approximated by inspection of structure. Wave functions of planar molecules can be multiplied with one another graphically or, in the mind's eye, to yield transition electric and magnetic moments. The gyration tensors have just one independent component. Transition dipole moments are orthogonal to one another. And, the most optically active directions are found at their bisectors. Throughout, emphasis is on the evaluation of long wavelength optical rotation, consistent with quantum chemical computation, using simple models that are part of the fabric of organic chemistry pedagogy.
非手性共轭烃的旋光率和旋光强度的计算旨在确定π→π*旋光强度的总和在多大程度上足以解释非共振光学活性。σ和π电子的可分离性可能为某些情况下手性结构-性质关系的解释提供捷径。研究表明，通过将分析限制在平面、C(2v)对称的π体系及其单电子 HOMO-LUMO 激发态，由于以下原因，对旋光性这一令人困惑的性质可以获得直观的理解：Hückel 波函数的计算非常简单，在某些情况下，甚至可以通过对结构的检查来近似。平面分子的波函数可以通过图形相乘，或者在脑海中相乘，得到跃迁电偶极矩和磁偶极矩。回转张量只有一个独立的分量。跃迁偶极矩相互正交。并且，最具光学活性的方向位于它们的等分线上。自始至终，重点都是使用简单的模型来评估与量子化学计算一致的长波长旋光率，这些模型是有机化学教学法的一部分。
